Light emitting diodes (LEDs) are often driven with a square wave signal that turns the LED on when the signal goes to a high voltage and off when the signal goes to a low voltage. One application for LEDs is to provide illumination in a scanning device as part of a contact image sensor (CIS). A contact image sensor typically includes an array of optical sensors and red, green, and blue illumination LEDs. The contact image sensor is held in close proximity to the media being scanned, in contrast to other scanning systems that utilize mirrors and other optical components to channel light to the sensors. Contact image sensors allow for higher power efficiency than is typically achievable with other scanner technologies.
The intensity of the light from the LED is dependent on the duty cycle and current level of the square wave signal with higher duty cycles and current levels providing more light. Unfortunately, using a square wave signal to drive an LED may produce undesirable effects such as radiated emissions and ground bounce. Undesirable radiated emissions include radio frequency interference (RFI) where steep rises and drop-offs in the square wave currents cause the RFI spectrum to widen and spread to undesirable frequencies. Ground bounce is the presence of a voltage potential on a circuit board ground conductor that is intended to have a voltage potential of zero. Ground bounce is often caused by a combination of conductor impedance, which includes reactance, with a rapid influx or abrupt termination of current and may affect the operation of other circuit components connected to the ground conductor.